1. Field of the Invention
The invention relates to a new process for the purification of water containing organic constituents by means of absorber resins and to preferred applications of the process.
2. Description of the Related Art
An adsorptive process is often employed for the purification of ground waters, industrial waste waters or process waters. In so doing, the water to be purified flows through a bed comprising granular adsorbents, leaves the undesired constituents adsorptively bonded on the adsorbent and issues purified out of the bed. Frequently the undesired constituents of the water are organic compounds, which are removed with the aid of polymeric adsorber resins.
Industrial waste waters or process waters, as defined in the above description, are all streams or quantities of water that are related in some way to industrial manufacturing plants or manufacturing processes and that consequently are contaminated with undesired constituents, be it because they themselves are, for example, components of a manufacturing process or that they make only indirect contact with manufacturing processes or plants, for example rain water from tank or installation bays. Such waters are often contaminated with environmentally unacceptable pollutants, often with organic compounds. The object is to remove these substances, except for a small residual concentration, as early as possible in the processing sequence.
By ground waters is understood all forms of ground, surface, rain or seepage water and the like.
In "Neuere Verfahrenstechnologien in der Abwasserreinigung, Abwasser- und Gewasserhygiene", R. Oldenbourg Verlag, Munich, Vienna 1984, pages 235 to 251, several methods for treating waste water, containing chlorinated hydrocarbons, from chlorinating plants are described. According to one method, the waste water is first made alkaline (pH 11). The subsequent stripping, which is done by blowing in water vapor in the countercurrent, results in a distillate that, following condensation, decomposes into a chlorinated hydrocarbon phase and an aqueous phase. The bottom of the stripper (oxistripper) collects waste water, which is then purified biologically.
In special cases waste water, having a pH value ranging from 1 to 2, can also be subjected to stripping with water vapor in the countercurrent. The bottom of this stripper collects water with a high iron and hydrochloric acid content, which is fed to a chemical-mechanical clarification plant for further processing.
In the above-mentioned literature the rain water, containing chlorinated hydrocarbons from plant and tank bays, is purified in an adsorber resin installation. In so doing, the rain water, from which solids have been carefully removed, passes through an adsorber resin, based on a styrene-divinyl benzene copolymer. Then vapor desorption takes places for reactivation. Based on the fact that after application of this process a significant reduction of the adsorber resin activity could be recorded after one year owing to irreversible iron deposits, a new plant with a pH of 1.5 was operated.
According to Technische Mitteilungen, vol. 77 (1984), 525-526, styrene copolymers or polyacrylates are suitable for removing aliphatic and aromatic hydrocarbons, chlorinated hydrocarbons, phenols, pesticides and surfactants from waste waters. Thereafter the resin is reactivated with water vapor. In the case of pesticides, however, reactivation occurs with the aid of solvents, like acetone or methanol, or with inorganic chemicals.
In the DD 249 190 hydrochloric acid, which is contaminated with organic substances and which originates from chlorination processes, is purified with the aid of hydrophilic, postreticulated styrene-divinyl benzene copolymers, which exhibit preferably an interior surface ranging from 1,000 to 1,600 m.sup.2 /g. The resin is reactivated with 130.degree. C. hot water vapor, whereby the bonded organic substances can also be desorbed with solvents, like alcohols and ketones.
The DE-P 44 33 225.4 discloses a process for the purification of ground water of organic components, where, first of all, solid components are removed and then the organic components are adsorbed onto an adsorber resin. Following desorption of the organic components with water vapor, the adsorber resin can be regenerated, if desired, with an acid. Suitable adsorber resins are preferably styrene-divinyl benzene copolymers with a specific surface ranging from 600 to 1,200 m.sup.2 /g. Desorption with water vapor is done preferably at 100.degree. C. to 150.degree. C. The desorbate, produced from desorption with water vapor, is collected and can be subjected to a phase separation into an aqueous and an organic phase. The separated aqueous desorbate phase can be cycled back and added to the ground water to be purified; the organic phase is extracted from the process. Especially suitable methods for removing inorganic deposits on the resin, caused by inorganic constituents in the water, are known from the DE-P 44 33 225.4.
Finally the DE-A-42 04 573 describes an adsorptive process for the purification of contaminated ground water, seepage water, soil washing water and surface water, in which process the adsorber is supplementally heated by means of the walls of the apparatus or suitable installations; this patent application describes suitable adsorber resins.
In addition to ground water purification, the adsorption of undesired organic constituents by means of reactivatable adsorber resins is also applicable to industrial waste water or process water purification. The goal of such a waste water or process water purification is to reduce the organic constituents in the water to such an extent that the water thus purified can be reused directly or released in a state that is compatible with the environment. Suitable adsorber resins are, for example, the aforementioned styrene-divinyl benzene-based copolymers (e.g. Purasorb AP 250 from the Purolite Corporation or Amberlite XAD-4 from the Rohm and Haas Corporation).
If one looks at only one adsorber at a time, then adsorptive water purification always takes place in cycles, irrespective of the total process. In the adsorption cycle the adsorber resin is loaded first. In so doing, three zones in the adsorber resin can be distinguished in the flow direction: the saturated zone, where the water to be treated enters the adsorber, in which the resin has reached the equilibrium loading, corresponding to the untreated water; the actual adsorption zone, where the loading of the resin declines from the equilibrium value to the residual concentration, remaining following desorption, in the resin (thus the fresh zone value); and the virtually unloaded fresh zone before issuing from the adsorber. As the loading increases, the adsorption zone migrates in the flow direction to the end of the adsorber bed. The width of this transition zone is a function of varying parameters, such as the flow rate of the water to be treated through the adsorber. When the fresh zone is consumed, the adsorber threatens to reach a breakthrough point and the adsorption cycle is ended. The adsorber resin has to be desorbed.
Desorption can take place by different methods, e.g. with the aid of suitable solvents. Another possibility of reactivation, which is preferred for water purification, is to rinse the adsorber bad with water vapor. In so doing, the adsorptively bonded pollutants pass into the water vapor, which is then condensed. With the majority of known applications desorption takes place in this manner with the water vapor ranging from 100.degree. C. to 150.degree. C. To conserve the resin, temperatures are set preferably from 110.degree. C. to 140.degree. C.
In this case the adsorber is usually loaded with demineralized water in order to displace the water to be treated from the resin. Furthermore, the demineralized water is usually continuously heated, especially in plants without supplemental heating of the adsorber, thus mildly raising the adsorber resin to about the desorption temperature. After water vapor desorption the adsorber bad is cooled with demineralized water of falling temperature, in order to avoid damaging the resin by a sudden change in temperature during the cooling process.
It is well-known that the added demineralized water is to be added to the water to be treated. After leaving the adsorber, the water vapor used for desorption is condensed and usually cooled to approximately 35.degree. C. to 25.degree. C. If the desorbed quantity of undesired organic water constituents is greater than can be dissolved in the liquid desorbate at this temperature, two phases are formed (a dry organic phase and an aqueous phase, saturated with organic constituents), which as a rule can be easily separated owing to their different densities. Therefore, the cooled desorbate is usually fed into a phase separator, where gravity separation into an aqueous and an organic phase occurs. The organic phase denotes the concentrated pollutant freight and can be extracted. In contrast, the aqueous phase has to be subsequently treated. It is well-known that this phase, too, is added to the water to be purified and is thus cycled back onto the adsorber bed.
The preparation of the saturated, but pollutant-lean aqueous phase, which accumulates during reactivation, is a critical point that decides the feasibility of the adsorptive water purification process. In the case of contaminated ground water, for example, the loading with undesired constituents is usually far below the saturation limit, so that the admixture of the constituent-lean (but saturated) water phase from the reactivation means a significant dilution of this phase and thus a loss of energy that has an adverse effect on the cost.
Furthermore, the concentration of the undesired constituents, for example, in a ground water, especially toward the end of a purification measure, can be so low that no two phases form in the condensate of the reactivating vapor. The ratio of the volume of constituents, separated off from the raw water, to the volume recirculated in the condensate, becomes increasingly worse. Then an adsorber, which is not reactivated following exhaustion, but rather is disposed and replaced by a fresh one, is often more economical.
The described conventional method of desorption with water vapor also fails when, for example, the adsorbate itself is relatively readily water-soluble and even when subjected to intense cooling the condensate of the reactivating vapor does not separate automatically into am aqueous and an organic phase. Of course, a mass transfer could still be attained by suitable methods (e.g. stripping and the like); yet the cost of such a separating method would have a significant negative effect on the feasibility of adsorptive water purification.
Therefore, to be able to avoid even in such cases the use of other expensive mass transfer or even desorption techniques (e.g. the use of solvents inclusive of the requisite after-treatment steps), the object of the present invention is to improve the phase separation behavior of the desorbate from the water vapor desorption by means of suitable process steps, in order to design adsorptive water purification processes so as to be applicable and/or more economical without additional expense, and especially when the undesired organic constituents in the water to be purified are relatively readily water-soluble or the degree of contamination is (still) quite low.